This mirror, to maintain its accuracy, not only requires careful mounting in its cell but also protection against temperature changes. Even though 12 inches thick it would bend under its own weight of 4,300 lbs. sufficiently to affect the figure and consequently it is supported in the cell by a specially counterweighted lever system so that it is equably supported at twelve points and there is no tendency to bend. A similar lever support system around the edge prevents distortion due to constraint when it is tipped from the horizontal position at different positions of the tube. Temperature changes can produce much greater distortion than flexure but Victoria has the advantage of very low diurnal range and the temperature change around the mirror is made very small by a lagging of cotton felt about 2 inches thick all round the sides of the closed section of the tube, laced on with a duck cover (compare Fig. 3 with Fig. 2) and an equal thickness below and around the edge of the mirror. By this lagging the temperature rise in the day-time is only about half a degree while the dome temperature increases five degrees, hence the figure of the mirror remains good whatever the temperature changes outside. The Principal Focus As already indicated, the principal mirror when used alone forms an image of the star 30 feet above, at the centre of the upper end of the tube, and an eyepiece could be placed there for visual work or a photographic plate for direct photographs of nebulae, etc. However, it is generally more convenient to use a flat mirror at 45° forming the image at the side of the tube for photographs and so the telescope is only used in this form with a small spectrograph for the ultra-violet region of star spectra. The course of the parallel beam of light from the star to its image on the slit of the spectrograph is graphically shown in Fig. 5 A and also its passage through the slit prisms and lenses of the spectrograph. The position of the star image on the slit of the spectrograph can be observed by a guiding telescope extending to the edge of the tube and can be kept central by the portable aluminium switchboard already described. The elevating platform is of course used in work in this position. The Newtonian Arrangement